Girlfriend’s best friend is someone she has slept with

So my girlfriend’s best friend is someone that she has slept with on and off her whole adult life essentially, anytime she hasn’t been in a relationship. She was up front with me that she has slept with her best friend before we started dating. What bothers me is that it still feels like there’s a lot of physical attraction there between them. She has several other male friends that I have met and there isn’t the same feeling I have with this dude that she has slept with. Like their texts feel borderline flirty and always sending sexually perverse memes.

Anytime I try to communicate something bothers me and it involves this dude she immediately gets super defensive and it always starts a fight. Even if it’s just wanting reassurance it doesn’t end well. Which is also different from her other male friends.

I feel like in the back of her mind she has to know it’s not a normal friendship because they really haven’t talked nearly as much as before we were dating. Not every day like before she was dating me.

Now we are at the point I feel we are more attached and she’s wanting to go see him again (she’s really only talked on the phone or texted him while we’ve been together) and the fact that the last time they hung out they slept together terrifies me. Am I insane for this bothering me so much? I feel so gaslight anytime I mention the dude and that I’m being completely insecure, so I can’t ever talk about it.